1. Field of The Invention
The present invention relates to a passive structural damping system for a wind turbine blade and, in particular, to a rotor blade for a wind energy turbine comprising a damping system for damping at least the first edge-wise structural bending mode of the wind turbine blade.
2. Related Prior Art
The blades of the rotor of a wind energy turbine are subjected to diverse edge-wise structural bending modes, meaning that the rotor blades are oscillating within the plane of rotation. Among these bending modes the first edge-wise structural bending mode typically has a merely poor structural damping effect (about 1% only) resulting from a typical blade construction. The first edge-wise structural bending mode typically has a frequency from 1-5 Hz and can be easily excited by wind turbulences. The first edge-wise structural bending mode can induce dynamic loads to all the components of the drive shaft of the wind energy turbine such as high blade root bolting dynamic loads at resonance frequency, high dynamic torque variation, increasing gearbox gear stages and bearings wear, and possibly high dynamic loads to the main bearing and bedplate, generating additional fatigue.
It is basically known to provide structural damping of the first edge-wise bending mode by means of a moving mass at the tip of the blade. U.S. Pat. No. 6,626,642 suggests to provide an oscillating mass on an articulated shaft connected to the shell of the blade by means of a specific loss bearing. WO-A-95/21327 provides a classical mass-spring-damper arranged within the tip portion of the shell of a rotor blade.
The known damping systems are complicated in construction and difficult to integrate into the internal construction of a rotor blade. Accordingly, there is a need for a damping system which is easy to manufacture and install, provides robustness and is failure-free and, accordingly, substantially maintenance-free.